The present invention relates to boom lift vehicles and, more particularly, to a boom lift vehicle including a tower boom pivotally coupled with a main boom and a method of controlling lifting functions of the boom lift vehicle.
In designing a boom lift vehicle, vehicle weight is an important consideration affecting manufacturing costs, vehicle maneuverability, safety factors and the like. Boom lift vehicles including one or more articulated booms typically include a strategically-placed counterweight in order to balance moment loads resulting from positions attainable by the boom arms.
Boom lift vehicles are known that include a tower boom pivotally coupled to a vehicle base. The tower boom may also be capable of expansion and retraction via telescope sections. Typically, with conventional arrangements, when raising the tower boom, the tower boom with its telescoped sections fully retracted is first pivoted to a max angle and subsequently extended from the max angle to a max position by extending the telescope sections. By raising the tower boom in this manner, a main boom supporting a platform and pivotally coupled to an upper end of the tower boom may be placed in positions that create a large turning moment. To accommodate such moments, the vehicle must include a large mass counterweight to stabilize the machine. Such larger counterweights, however, increase manufacturing costs and may have a detrimental affect on operating envelopes, for example, when the vehicle is operated on an incline. Additionally, vehicles exceeding a certain weight limit require special permits for transporting via public roads. This added consideration results in still higher costs to the vehicle purchaser.
In previous arrangements, forward stability positions are most critical when the main boom is extended near a horizontal angle and when the tower is fully raised in angle but fully retracted in length. Backward stability conditions are most critical when the main boom is fully raised when the tower is lowered and retracted or when the tower is fully raised and fully extended. Allowable positions of the tower other than these end points gain backward stability margin at the expense of forward stability margin as described above.
An articulated machine typically includes an upright and a means to maintain the upright in the vertical position when raising the tower either by an upright level cylinder or mechanical linkages. This is done to transfer the reference angle of the turntable or ground for platform leveling, to reduce the total stroke of the main boom lift cylinder and to avoid the main boom lift cylinder from having the capability of positioning the main boom into positions of backward instability.
U.S. Pat. No. 6,488,161 describes advantages of using the tower and main boom as counterweight by limiting the positions of both forward and backward stability, particularly when the tower is raised from 68 to 72 degrees when the main boom is raised from 15 to 55 degrees. By reducing the horizontal outreach of the machine, a destabilizing moment of the upper boom and platform load is reduced. Such a construction also enables the weight of the boom structure to be in the most favorable position to aid in the counterbalancing of the upper boom and platform load destabilizing moment.
In previous machines, the working envelopes of the booms were mechanically limited. When these machines were operated on sloping ground, the ultimate angle of the booms was a function of the mechanical limits of the machine and the angle of the ground. This effectively tilts the working envelope by the actual ground slope, increasing and decreasing the reach of the platform from the base of the machine. The increased angles of the boom detracted from the stability of the machine and therefore resulted in the addition of counterweight.